Adaptive mesh refinement cosmological radiation hydrodynamic simulations pioneered by this group have recently expanded to include sufficient physics and enough resolution to simulate directly the prior effects of Population III star-forming mini-halos on the first galaxies. This technical advance has occurred just when observations from both ground and space are providing astrophysical data on galaxies in the previously unimaginable range of redshifts from 6 to 8.5. These observations indicate substantial evolution in the bright end of the luminosity function, and a steep faint end slope consistent with the notion that high-redshift dwarfs dominate the reionization photon budget. They also reveal substantial evolution in stellar populations over this redshift interval. The advance in simulation capability is also well timed for the start of operations of the first NSF-supported sustained petaflops supercomputer, Blue Waters.
Combining these software, hardware, and data advances, this project will simulate the formation and evolution of the first galaxies across the entire range of masses and luminosities observed, and compare the results to the data through simulated images and spectra. This will lead to inferences about the Population III initial mass function, feedback effects, the growth of supermassive black holes, and even the dark matter power spectrum, and place important constraints on the accretion rate of gas onto galaxies. The simulations will improve estimates of the escape of ionizing radiation into the intergalactic medium, and directly address whether high-redshift galaxies are capable of reionizing the Universe. This is the very first time such near-first-principles simulations are feasible, due to the convergence of petascale computing hardware, simulation software, and algorithmic advances, and also the first time that relevant observational constraints are being obtained.
Simulations at this scale have a strong impact on the astronomical community, the high performance computing community, and the larger science and technology-curious public. They drive the development of the hardware, software, and networking infrastructure. This project is one of six selected for first access to NSF's new petascale supercomputer, and the required code enhancements will be made generally available. There are also plans to include these scientific results in a new IMAX film by the Reuben H.Fleet Science Museum in Balboa Park, San Diego, anticipated in 2013.
This project was designed to address a pressing need in computational astrophysics research: a highly robust, accurate and freely-available code for simulating radiation transport processes of the early universe, and that may be used on the largest NSF supercomputers available today. To that end, this collaborative project between astrophysicists at the University of California San Diego (UCSD) and computational mathematicians at Southern Methodist University (SMU), focused on the construction of open-source software for these physical processes within the "Enzo" community cosmology simulation code (http://enzo-project.org). This work follows a previous NSF-funded project, in which we had constructed an initial diffusion-based model for radiation transport. Our previous model focused on supercomputer-scale algorithms, using a simple single-frequency radiation transport model (e.g. only one color of light), and assuming that the entire universe was equally resolved within the simulation. Our specific aims in the current project focused on pushing the limits of this previous model to support multi-frequency radiation propagation (i.e. multiple colors), adaptive mesh refinement (allowing us to "zoom in" on physically interesting regions of space), and to interface with a much more computationally costly but increasingly realistic ray-tracing model for radiative transfer. The end result of this project is a free and easy to use code that combines the best features of both approaches: simulations with high spatial and angular accuracy around early galaxies, more realistic resolution of the types of radiation that emit from these galaxies and change over time and distance, and robust algorithms that more efficiently utilize publicly-funded supercomputing resources. This SMU portion of the project focused on the mathematical models, computational algorithms and software encompassed by this project. Our UCSD collaborators have been using these solvers for astrophysical studies of the early universe, which will be elaborated upon in their report. Our project has not only proved beneficial to theoretical astrophysics. Many simulation tools across the scientific and engineering spectrum utilize similar adaptive-mesh computational techniques as Enzo. Many of these simulation tools currently include, or plan to implement, physical models that are mathematically similar to our diffusion-based radiation transport model. In developing our codes for this project, we invented new solution algorithms for problems of this type that are based on these adaptive mesh techniques. While these new mathematical algorithms currently reside only within this simulation code, we plan to help other scientific communities to adopt these new algorithms as well. Finally, the research funded by this project has been disseminated within both the computational astrophysics and applied mathematics communities. Specifically, it has contributed to four academic journal articles (three already accepted for publication and the fourth in review), seven open-source software releases, and three presentations at national and international workshops and conferences. Additionally, it has partially funded the graduate education of multiple astrophysics students, as well as tutorial sessions on advanced computational mathematics algorithms for the broader Enzo community.
